The present invention relates to dc motor controllers and, more particularly, to control systems for dc motors having separately excited armature and field windings.
Material handling trucks fall into one of several power plant categories. One such category is the electric vehicle, the energy source for which is a lead-acid battery that can weigh many thousands of pounds. Besides providing the energy source to the vehicle, in many instances the battery also provides vehicle counterbalance.
The ratio of the load weight to the gross unloaded vehicle weight of industrial lift trucks is extremely important. For example, if an unladen vehicle weighs 12,000 lbs, and the maximum load weight it can carry is 4,000 lbs, then the gross unladen/laden weight may vary from as little as 12,000 to as much as 16,000 lbs. This represents a change of 33% in motor torque requirements. Moreover, the vehicle must be able to maneuver on loading ramps, further increasing the motor torque requirements. For these and other reasons, it is desirable to have a control system capable of extracting precise and efficient work from the vehicle.
The main motive element of this type of vehicle, referred to as the traction system, conventionally consists of a series-wound dc motor coupled to a gear reducer and drive wheel. Some electric vehicles utilize a single "steer-drive" traction system, while others employ a "dual-drive" (differential) traction system.
The rotational direction of the series-wound dc motor is controlled by the polarity orientation of the field winding with respect to the armature. Under conventional control, the field winding orientation is controlled through a pair of contactors, such that when power is applied across the field-armature combination, the motor is caused to rotate in the desired direction.
The series-wound dc motor, heretofore used extensively in industrial lift trucks, displays one very important characteristic: it has extremely high torque at zero speed. This is extremely important, because it provides the necessary starting torque.
Under conventional control, the field-armature combination is controlled as a single unit. Motor speed regulation is achieved through voltage switching typically utilizing such power semiconductor technologies as silicon-controlled-rectifiers (SCR). The voltage drop associated with the SCR as well its duty cycle limit impose a speed limit on the motor. To extract the maximum speed from the motor and reduce overall system power loss, a bypass contactor is utilized across the SCR, thereby placing the motor's field-armature combination in series with the battery.
Under such a control scheme, however, the series dc motor does have one major drawback: it may operate only along its characteristic commutation curve limit. This results in motor speed variations due to changing torque loading arising from variations in load capacities, travel path conditions and grade variations.
With the proper controls, the use of a shunt-wound dc motor under independent field and armature control can provide distinct advantages over conventional series-wound dc motors for lift truck applications. The control method of the present invention provides the shunt-wound dc motor with the ability to simulate a series-wound dc motor, hence developing the necessary starting torque.
The separately excited dc motor represents a highly coupled multi-input, multi-output, non-linear, dynamic system or plant. It is highly coupled in the sense that, when one of its inputs is changed, all of the outputs are affected. This is undesirable, since the purpose of control is to knowingly and intentionally affect the desired output(s) only, without altering other output states.
U.S. Pat. No. 4,079,301 issued to Johnson, III discloses a dc motor control circuit having separately excited armature and field windings. The control circuit is operable in both the constant torque and constant horsepower modes. The transfer characteristics of the circuit provide high gain at low frequencies and low gain at higher frequencies. The circuit can further reduce the gain at low frequencies when motor operation switches from the constant torque mode to the constant horsepower mode.
U.S. Pat. No. 3,694,715 issued to Van Der Linde et al. discloses a contactless dc motor reversing circuit. The current from a variable frequency, pulsed dc source is applied to the series field by a pair of solid state switching devices for forward motor rotation. A second pair of solid state switching devices applies current for reverse motor rotation. Common to both switching devices is a third switching device which normally carries the induced armature current between pulses. It is de-energized during transfer of conduction between both pairs of switching devices, assuring that the blocking state of one pair occurs before the second pair is turned on.
U.S. Pat. No. 4,264,846 issued to Sauer et al. discloses a speed control braking circuit for a dc motor. The field and armature currents are independent of each other to allow motor operation in the field weakening region. The armature current is set by a pulsing dc element. The field winding is contained in a series circuit with a switch which is connected in parallel with the dc element. Shunted across the field winding is a field current bypass diode.
It would be advantageous to provide a motor control system capable of producing variable torque while maintaining constant speed.
It would also be advantageous to provide a system in which the characteristics of a series-wound dc motor could be simulated using a shunt-wound dc motor.
It would further be advantageous to provide a system in which a traction motor's field and armature windings are separately excited and controlled.
It would also be advantageous to provide a system in which the motor can be controlled by a decoupling controller.
It would still further be advantageous to provide a system in which the decoupling controller is achieved using software.